The principle of transmission in FSK radars is known. As can be seen in FIG. 1, it consists in sending a repetitive pattern constituted by a wave that is frequency-modulated in plateaux or steps.
Furthermore, if T is the repetition period of the pattern, the ambiguous distance D is defined in a known way, by the formula:                     D        =                              C            ·            T                    2                                    (        1        )            
The link budget of the radar is generally sized in such a way that, for the majority of the types of targets encountered, the power of the signal backscattered by targets located beyond the ambiguity distance is below the detection threshold of the radar. However, in certain particular circumstances, the signal backscattered by targets located beyond this distance is above the detection threshold. This is the case for example when the radar directed at a specifically chosen angle. Echoes with erroneous positions are then obtained at output of the radar receiver, and need to be eliminated.
To limit these undesirable echoes, it becomes necessary to increase the ambiguity distance of the radar by increasing the period T of repetition of the pattern sent. Increasing T amounts especially to increasing the number of stages or again to lengthening their duration.
This approach however is not always applicable because the sizing of the various parameters of the radar is generally the result of a compromise on the waveform emitted. This leads for example to limiting the number of plateaux or again their duration.